This invention relates to machine tools and specifically to tools and machining methods for microfinishing cylindrical workpieces such as crankshaft journals to a high degree of precision.
Numerous types of machinery components must have finely controlled surface finishes in order to perform satisfactorily. For example, surface finish control, also referred to as microfinishing, is particularly significant in relation to the manufacturing of bearing journals such as are found in internal combustion engine crankshafts, camshafts and power transmission shafts. For journal type bearings, very accurately formed cylindrical surfaces are needed to provide the desired hydrodynamic bearing effect which results when lubricant is forced between the journal and the associated bearing. Improperly finished bearing surfaces may lead to premature bearing failure and can limit the load carrying capacity of the bearing.
Currently, there is increasing demand for higher control of journal bearing surfaces by internal combustion engine manufacturers as the result of greater durability requirements necessary to offer improved product warranties, the higher operating speeds at which engines (particularly in motor vehicles) are now required to sustain, and the greater bearing loads imposed through increased demand on engine structures.
Various types of microfinishing techniques are presently in use. In stone microfinishing, an abrasive stone is brought directly into contact with the surface to be machined. This process has numerous shortcomings for journal surfaces.
In another type of microfinishing process, herein referred to as conventional abrasive tape microfinishing, the part is rotated and an abrasive coated tape is brought into contact under pressure against the surface. As the part is rotated, the abrasive material reduces the roughness of the surface. In the conventional process, the tape is brought into contact with the part by pressure exerted by compressible elastomeric tools or inserts, typically made from urethane plastic compounds. This process overcomes some but not all of the disadvantages associated with stone microfinishing. Principal among disadvantages not addressed is the fact that the process does not adequately correct geometric errors in the part being microfinished, since the tool acting on the abrasive tape is a flexible and therefore, the tape conforms to the surface profile of the part which may have been inaccurately formed in prior machining operations.
Microfinishing equipment developed by the assignee of this invention, the Industrial Metal Products Corp. (IMPCO) provide a significant advancement in abrasive tape microfinishing. These machines are capable of precision microfinishing, both in terms of surface finish improvement and some aspects of geometric form control. This new generation of microfinishing equipment is referred as "Generating Bearing Quality" (GBQ) tools and processes and are encompassed by assignees U.S. Pat. Nos. 4,682,444; 5,095,663, and 5,148,636 which are hereby incorporated by reference. This new approach employs an abrasive coated film made of a polyester material such as that manufactured by the 3M Company. The film is pressed against the workpiece using rigid tools having a roughened surface such as made from honing stone material or textured metal. The surfaces are accurately formed and are substantially non-compliant. Therefore, the precisely formed configuration of the tool surface is impressed in the workpiece surface and thus certain types of geometric form errors are improved such as lobbing or other errors in circular geometry (i.e. departures from a true circle in a diametric cross-section through the part). The GBQ tooling also features a relatively large wrap-around angle or angle of engagement between the tool and workpiece which further enhances control of geometric imperfections and provides improved material removal rates. This tooling coupled with periodic reversing of the relative direction of motion between the abrasive coated film and workpiece provides the ability to remove significant amounts of material in an extremely accurate controlled manner.
Even with earlier GBQ technology, as described in the previously referenced patents, some geometric imperfections are not adequately corrected. In the processes discussed, the microfinishing tool is narrower than the length of surface to be machined and is axially stroked along the surface to generate the desired surface finish characteristics. This axial stroking produces a generally uniform material removal rate along the length of the surface.
Form imperfections in crankshaft journals (and other precision cylindrical surfaces) may include deviations along the axial length of the journal (i.e. along the central longitudinal axis of the journal referred to as "axial form errors"). For example, one axial end of the journal may have a larger diameter than the opposite end, referred to as a tapered configuration. In addition, "hour-glass" imperfections are sometimes encountered in which the axial center of the journal has a smaller diameter than the ends. An opposite form referred to as "barrelling" also occurs in which the diameter of the journal is greatest at its axial center. These types of axial form errors are not adequately addressed by currently available microfinishing machining tools and processes due to their generally uniform machining action along the axial length of the journal.
The tooling and processes according to this invention however, provide a means of improving axial form errors found in cylindrical surfaces such as bearing journals. The microfinishing tools and processes according to this invention feature a means of axially shifting the center of pressure exerted by the microfinishing tool which presses the abrasive coated film into engagement with the workpiece surface. In conventional machines, the axial pressure distribution along the face of the tool is symmetrical about the axial midpoint of the tool and is fixed. By changing the center of pressure of the tool, a controllable asymmetrical machining effect can be provided along the axial length of the workpiece surface. By creating a greater pressure closer to one axial edge of the tool, a greater material removal rate is provided in that area. Thus if the greater machining effect is caused to act where an enlarged diameter is found as occurs in a tapered journal, after microfinishing a configuration closer to a true constant diameter cylinder cylindricity or other desired configuration can be provided. Shifting of the pressure toward the opposite axial edge of the tool provides improvements in geometry for journals tapered in the opposite direction. By shifting of centers of pressure during machining hour-glass type form error can also be addressed.
In accordance with this invention, a novel microfinishing tool assembly is provided in which a shiftable or axially variable machining effect is provided. If this tool assembly is combined with in-process gauging capable of diameter measurements at various axial locations, precision in-process control can be provided which is highly adaptable to variations between parts.
Microfinishing tools and processes having the features of this invention are provided in two embodiments described herein. In the first embodiment, the pivot point of a pivot shaft which mounts a microfinishing tool shoe is mechanically changed, resulting in a shiftable center of pressure acting on the machining film. In another embodiment, an external torque is applied to the microfinishing tool shoe which also provides an axially shiftable machining effect.